It is known to combine the projection images into reconstruction groups in such a way that in the process each of the reconstruction groups includes all the projection images to which a specific one of the image times is assigned. Alternatively, each of the reconstruction groups includes all the projection images whose image times lie within a time interval specific to the respective reconstruction group. Each reconstruction group is defined in such a way that a three-dimensional reconstruction of the examination object with direction-independent local resolution can be determined on the basis of the projection images assigned to the respective reconstruction group. With regard to each reconstruction group, the respective three-dimensional reconstruction is determined. In addition, a reconstruction time is determined in each case on the basis of the image times assigned to the projection images of the respective reconstruction group and said time is assigned to the respective three-dimensional reconstruction. Further analyses are performed as a function of the temporal sequence of the three-dimensional reconstructions.
Analytical methods of this kind are usually performed using projection images which have been acquired by means of CT systems. In CT systems, a recording arrangement which comprises at least an X-ray source and an X-ray detector rotates at relatively high speed (currently up to 180 revolutions per minute) about a rotational axis. During the rotation the X-ray source and the X-ray detector are controlled and the projection images captured. The totality of captured projection images is subdivided into the reconstruction groups. For example, each reconstruction group can correspond to a complete revolution of the recording arrangement about the rotational axis. The corresponding three-dimensional reconstruction of the examination object is determined on the basis of the respective reconstruction group. The determined three-dimensional reconstruction has a direction-independent local resolution. The term “direction-independent local resolution” is to be understood in this context in the sense that the volume dataset has volume data elements which are localized in space by means of three coordinates of a Cartesian rectangular coordinate system. In this scheme each individual volume data element is embodied as a cube shape. In contrast thereto, the volume data element in the case of direction-dependent local resolution is embodied as a quadrangular shape.
The reconstruction time is, of course, assigned to the three-dimensional reconstructions in a meaningful way. For example, the average of the acquisition times of the corresponding projection images can be used. The further analyses comprise in particular functional medical evaluations, perfusion analyses for example. Other analyses are also possible, however.
In CT examinations, the examination space in which the examination object (typically a human being) is disposed is very constricted. Usually the examination object is not accessible from outside. If it is intended to perform interventions on the examination object, the examination object must be removed from the CT system. Only then can the intervention be performed. If further measurements are to be performed next, the examination object must be brought back into the examination space. Needless to say, the CT system must continue to be available.
Often a CT system is not available for image acquisition. In that case the image acquisition must be carried out using a conventional fluoroscopy apparatus, for example using a C-arm X-ray system.
In the case of C-arm X-ray systems too, the recording arrangement is usually movable. With many C-arm X-ray systems the recording arrangement can even be swiveled through 180° or more about an axis of rotation, with the result that the corresponding C-arm X-ray system can also be used to capture projection images on the basis of which—analogously to CT images—a three-dimensional reconstruction of the examination object can be determined which has a direction-independent local resolution.
With C-arm X-ray systems, however, the recording arrangement can be swiveled only relatively slowly. Depending on system, swiveling through, for example, 200° (more generally: 180° plus fan angle of the recording arrangement) takes at least four seconds, and considerably longer in many cases. This period of time is much too long for a multiplicity of possible—in particular medical—applications. In particular, for example, no three-dimensional perfusion sequences can be determined in this way.
In order to capture perfusion sequences using a C-arm X-ray system, in the prior art the recording arrangement is consequently positioned in advance and said positioning is maintained subsequently. Only on completion of the positioning is a sequence of two-dimensional projection images recorded. The further analysis is carried out on the basis of the two-dimensional projection images. A three-dimensional reconstruction is not determined and is also not possible with this approach. Only a DSA (DSA=Digital Subtraction Angiography) is possible.
An evaluation method for a number of two-dimensional projection images of a three-dimensional examination object is known from DE 103 54 214 A1. With said evaluation method, an image time and imaging parameters are assigned to each projection image. On the basis of the image times, a phase angle is determined in each case in conjunction with a phase signal of the examination object, said phase angle corresponding to a status of the examination object assumed at the respective time. The imaging parameters describe how the three-dimensional space has been mapped into the respective projection image. The projection images are combined into reconstruction groups. In this case each reconstruction group includes all the projection images that were captured within one and the same heart cycle within a predefined phase range. Based on the definition of the reconstruction groups, whereas a three-dimensional reconstruction of the examination object with direction-dependent local resolution can be determined on the basis of the projection images assigned to the respective reconstruction group, it is not on the other hand possible to determine a three-dimensional reconstruction of the examination object with direction-independent local resolution. With regard to each reconstruction group, the respective three-dimensional reconstruction of the examination object is determined. A plurality of three-dimensional reconstructions which originate from different heart cycles but are referred to the same phase range are combined with one another in suitable way in order thereby to obtain a CT section image, i.e. a three-dimensional reconstruction of the examination object with direction-independent local resolution.